Although the present invention is described below with reference to the determination of the soot and fuel concentrations in an aged motor oil, the present invention is not limited thereto, but rather generally relates to methods which use two physical variables to determine the quantity of two contaminants in a fluid. Motor oils are used in internal combustion engines to lubricate moving parts to reduce the friction and abrasion of metal surfaces moving relative to one another. Motor oil is subject to multiple aging processes which make it necessary to change the oil after a certain period of time. In this regard, it is extremely useful to provide a sensor device and a method which can detect the condition of the oil in an internal combustion engine during operation.
The condition of the motor oil is influenced in different ways by a wide range of aging processes. The effect of some aging processes on the properties and composition of the motor oil is known. Moreover, these aging processes may be identified by certain processes in an engine or equipment connected to the engine. Conversely, possible adjustment errors or defects in the motor or connected equipment may therefore be determined from the condition of the oil. This information is used in oil analyses which determine the composition of a used oil through complex physical-chemical laboratory process steps and, on the basis of this composition, enable conclusions to be drawn as to the condition of an engine. However, these oil analyses have the disadvantage that they cannot be carried out locally in a vehicle without a great deal of effort.